[esp32c6] mstatus doesn't change while single-stepping

[chrisnc]

I'm debugging some bare-metal code on an esp32c6 WROOM board using the latest released versions of riscv32-esp-elf-gdb (14.2_20240403) and openocd-esp32 (v0.12.0-esp32-20240318).

In, for example, an OS context switch, the assembly code must write to the mstatus register with something like:

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csrw mstatus, t0

before doing an mret into the new OS task.

What I notice is that when this instruction is executed by instruction single-stepping, the value of mstatus does not change, but if I set a breakpoint somewhere past this instruction and continue over it, the value of mstatus will change.

The simplest example I have to show this is below:

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    .section .text.start, "ax", %progbits
    .global start
    .type start, %function
start:
    csrs mstatus, 8
    ebreak

If I single-step through this sequence, the csrs has no effect, while if I continue through it, bit 3 of mstatus will be set as expected.
I'm wondering if something about the debugger is implicitly manipulating mstatus in an unexpected way.

Loading the program and continuing:

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(gdb) continue
Continuing.

Program received signal SIGTRAP, Trace/breakpoint trap.
start () at src/start/start.S:6
6	    ebreak
(gdb) p/x $mstatus
$1 = 0x88

Loading the program and single stepping:

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(gdb) si
6	    ebreak
(gdb) p/x $mstatus
$2 = 0x0